Noise operated automatic volume control



March 1211, 195o Lv H. GOOD NOISE OPERATED AUTOMATIC VOLUME CONTROL Filed DeC. 6, 1946 2 M W. un M MNM w. W ,A z i E i J Mz? W. M al? M i0 I u M e z e MW; m MQ 6mm f/m ya i ya, 3 z/ e i ...z c.. i u. w,

i.: fw w /F i r 25 www a z im Patented Mar. 21, 1950 fNoIsE oPERAf'rED AU'roMA'HavoLUME.

v y TROL I lowellHi Good; Riverton, N; J., assign'or t'oRadiol Corporation ofAmerica, .a corporationof Delaf.

CON

Warez' *A Appiicationnecemrer s, 194s; senarNo. 714,473.

This` invention relates* to anoise' operated auto vmatic controlv for'controlling. the output volumeV fromA one or more loudspeakers in accordancewith the noise level-in the vicinity off-the speakers.` Theoutput level of the' apparatus'v is set to operat'e at a predetermined'levelL above the level ofi the: ambient noise and, by means ofthe-automatic" control arrangements, asthe noise level varies-the'l output volume'varies similarly;

The invention` isci primary usein theatres,l

restaurants, auditoriums or other similar places;l

where variationsinthenumber of p'ersonslisteningto the sound varies from'ftimeto timeor Where" other conditionscause a variation inthe-ambient noise level such asinzmanufacturine plants andothen similar places. u

A. noise operated automatic volume control-1; similar to that described inV thepresent invention is.. described and claimed in U.' S; Patent No;

2,338,551;- issued January 4', 194.4, to reward--V0 Stanko.. While the arrangement' described'in-the patent operates to-cont'rolthe level ofthe sound produced by one or more loudspeakers in accordi;

ance with.. the. ambient noise level', it. has beenu s found that amore satisfactory control' can be obtained where, the power. output to the loudspeaker. measuredin decibelsi's;.caused to vary linearly and in directproportion with respectY to the power levelin decilzelsoithev ambient noise. If, as hasbeenheretoiore suggested, a bias voltage is produced which'is linearly relatedtothe ambient. noise. level. and utilized to control` the gain of .a variable gain amplier. tube,.it will be found that the above. mentioned.v characteristicy will not result. Thiais.duetolthefactthatl the.. output of a voltagerampliiier. oia.. conventional type is a logarithmic function of `the.a;;\plied.D.-(3. grid controllbiassf It'is therefore the primary object of this. inventionto .provide a noise operated automatic volume control in which the soundoutput of a loudspeaker. is varied in direct. propor tions. with, respect.- to the: variation of thet sound.

levelmfpambientxnoises..A Y

It isaafurther objectofthisinventlon to provide: an noise: operated. automatica volume control. in.` which the ambient noise:l converted: to= a: bias-i. voltage Whichf has.' a;. logarithmic .f relation to the.: noise' level infoi'deretoz. compensate:` ionnormal logarithmic output' of the".l variable' gairr. ampli#- iier' used` int the -circuit-whichi driveszthlloudfr Speakers.

Itis at still" furthersobjeot'# ofthis :invention-tov` provide a thermionic-amplier-in whichltlrieLout'M 5; Glaims'.. (Cl. 179-1)i asto vary linearly withtheY variation inthe ami bient -noise 1 level; It has further' been` determined that; shoul'dA thel noise'level' reach a very/'high value, it desirable toA increaseu the output' ofv theI loud'- speakersbeyonda' predetermined point, depel'idving vuponthe acoustics of'theroomand/oth'er conditions. It is thereforea stilly further object off` this invention to provide a' noise operatedl auto"- V'matie' volumeV control in which' the' maximum sound' output may be limited: to apredetermined" value:

The-'novelt features thatl are considered characteristic-of this invention are 'set'forth' With'particularityin= the appended. claims; The inven-v tion` both as to its organization' and method" of operation, as well asA additionalI objects` and advantages thereof', Willib'est he understoodv from4 the followingdescription' Wherr read in connection-with theaccompanyingdrawings; in-which Figure. l isa. block diagram'illustrating thisinv vention,

DlfPigure 2" is a circuit' diagram illustrating an" embodiment ofthisinvention; andf Figure 3' is a series or curves' showing the re-v lationship between thenoiseinput and the signal output `of the 'system described;

Referringto Fig: 1, anysuitablesource'offaudio* signal' l Which may be' a voice-actuated microi-v 'phone radio set or' phonograph reproducer; is

connected.` to the input of a` variablegain ampli--V iier`3. The output of the variable-gain amplieris" connected, if." necessary; to an" additional. volt-- age-a1np1i1ier5Y and' thenl toa power amplier 1i more loudspeakers 9*" inv the conventional man-'- ner;- Located* in the same room with theloudf speakers 9"^is a pickup microphone l I. In order toavoid undesired feedback' between the loudlspeakersand the microph'onetlre latteris prefere ably mounted adjacent" the' primary source of noise,- andmay beprovided with'soundabsorbing baiiles i3' andi I Svvhich' are-so-positionew With-re'- spect-` tothefmi'crophone H" 'ast tosubstantially shield'- itv from tlie= directfra'diations from tlieV loudspeaker. Theoutput` of the microphone: is: applied tola@ logarithmic* ampliiier l1 and; if; necessary; to a voltage=amplifler^l9 the outputofl which is" appliedtoa' rectifier' ZL The recti edvD.-C. outputoifrectiiier 211 isthe-bias vo1tageiwhich is applied vto the'. variable gain: ampli-y er'3; In addition a1 limiter' 23'fis connected;l circuit' with reetifi`er2l,: in Aorder to, limit the.: amplitude: of the-DC2 biasvoltage. tot azprede'v put level meesuredin decibels will be controlled soi 55p termined value.

ground through a resistor 53.

the limiter, and its cathode is connected to the..

.JROSS REFERENCE the 6K7 type, the-input circuit including a yre-v sistor 21 by-passed to ground in order to pro` vide a complete circuit for the input audio voltage. nected to the biascontrol source 2| which will be described subsequently. The output of the amp1i er tube 25 is coupled to a potentiometer gain control 29 which feeds the voltage amplifier 5.- and in turn the power amplier 'I to. which the..

speakers 9 are connected.

The noise operated volume control circuit4 comprises the microphone ll which is connected through an input transformer 3i to the anode electrode of a diode 33, the cathode of which is returned to ground through a resistor 35. The

sistor 37 to the grid of an ampliiier tube 39 which `may be of the 6J'7 type, for example. The cathode of amplifier tube 39 is connected to ground through, a Variablev resistor 4| and to a suitable source of positive potential through a-series resistor 43 of high resistance value. The output Acircuit of the amplifier is coupled to a potentiometer 45 and is provided with D.C. plate potential in the conventional manner.. A voltage amplifier #tube 41 has its grid connected to potentiometer 45 and its platecoupled to the anode electrodes of two diode rectiers 49 and5i. The former constitutes a rectifier for convertingthe A.C..

voltage representing the ambient noise into a D.C. potential, the amplitude of which is logarithmically related to the noise level. This voitage is derived by a connection tothe cathode of rectifier 49, the cathode also being connected. to Rectifier 5I is movable contactof a potentiometer 55, thefresistance element of which is connected between ground and a suitable source of positive potential I in series with a fixed resistor 51.

The circuit is rst adjusted by setting cathode resistor 4l of the .amplifier tube 39 to such a value that the effective grid voltage is approximately at the point Where grid current just begins to'flow. This adjustment is made in the absence of; a noise input signal.. The'setting of this re sister controls the slope of the input-output curve. Increasing the positive cathodebias voltage increases the slope of the curve and decreasing'I the bias decreases the slope of the curve. The normal operating range is, for the type of tube indicated, between -l-.4and i-LO volts, Which-corresponds .to a negative grid bias of the same range. The actual value required to cause the audio output level to be directly proportional to the noise input level can best be determined by making a performance test ofthe actual amplifier. v h. V.'Ihe purpose of rectifierl 33,l connected in4 th Y input circuit to the amplifier tube 39, is to remove the negative half ofthe appliednoise-representing signal so that the tube-A operates only with a positive grid voltage. This assures that a small grid current flows in the tube at all times and, as` a result, the plate current will havea logarithmic relationship to the amplitude of the noise signal .applied by the microphone H. Rectifien 33,., ampliertube 39.4 and v`the4 associated circuit elementsk together constitute a logarithmic l amplier.

The lower terminal of resistor 2 1 is con-- The A.C. noise-representing signal having a logarithmic relationship to the ambient noise, as described above, is then amplified by a conventional voltage amplifier tube 47 which needs no further description. 'This voltage is then rectified and filtered by the diode rectier tube 49 to produce a D.C. voltage whose' amplitude is in urr; a logarithmic function of the ambient noise eve 1 Since, as stated above, it is necessary to apply tol the grid of the variable gain amplifier 3 a control bias which varies logarithmically with the ambient noise in order to causethe output of the `loudspeader to vary linearly with respect to the noise; level, the D.C. voltage appearing across resistor 53, whenv applied to the control grid of tube 25, will produce the desired result.

In order to limit the maximum amplitude of the f output the potentiometer is adjusted so that a given positive potential is applied to the cathode of the rectiiier 5I. For low levels of input the platev of diode 5l will be less positive tl'ianthis` predetermined value 4and the tube will presentl a high impedance across the associatedr rectifier I 49 and willnot interfere with its operation. However, whenthe level ofthe control bias equals or exceedsv this predetermined value, the limiting rectifier 5l will present a low impedance to the associated rectier 49 and will effectively` limit the bias voltage to any desired predetermined' value.

Fig. 3 shows a` series of performance curves' in whichthe signaloutput, shown as the ordinata. isplottedagainst noise input, shown as the abscissa, buth input and output being measured in decibels, for various settings of the limiter,

.the curve for 2.5. volts D.C. being the input vs.`

output characteristicof the apparatus when the positive. bias applied to the limiting diode 5i has valueof 2.5 volts and limiting begins at a low output level.. The other curves show the variation of, signal output as a function of the noise input as the bias on the control diode 5i is varied through a number of values up to 10 v. D.C.

" It can be observed that for thev 10 volt curve, for

example, the output is substantiallylinear and is directly proportional to the input between a region from. -20 to -40'db. input'. This is the normal operating range and it is therefore evidentV that the device operates in the required manner to vary the signal output as a linear function oi the noise input.

I' have thus described a noise operated auto-A matic volume controlwhich samples the ambient noise level in the vicinity of a loudspeaker and which varies the loudspeaker output automati cally as a linear function of the noise level.A

What I claim is:

1; In combination, a variable gain amplifier; a loudspeaker operated by the output of said amplifier, noise pickup means, means in circuitwith said noise pickup means for. producing a D.C. bias voltage which varies as a logarithmic function of the output of said noise pickup means, and means for utilizing said bias voltage to vary the gain of said variable gain amplifier, thereby to varythe output power level of the loudspeaker as a linear function of the ambient noise level.

2.. In combination. a source of audio frequency voltage, a variable gain amplifier having control grid and plate electrodes for amplifying said. voltage, said amplifier having an output voltage characteristic which is a. logarithmic function of an applied D.C. biasing potential, a power amplier. coupled torsaid variable gain amplifier and including output means for producing an audio sound output corresponding to said audio. frequency voltage, noise responsive means for producing an alternating voltage proportional to the amplitude of ambient noise in the vicinity of said sound output means, means for converting said alternating voltage into a D.C. bias voltage which varies as a logarithmic function of said ambient noise level, and means for applying said bias voltage. to said control grid to control the gain of said variable gain amplier, thereby to vary the output power level of the sound output means as a linear function of the ambient noise level.

3.. A device of the character described in claim 2 which includes means for limiting the maximum amplitude of said bias voltage.

4. In combination, a source of audio frequency voltage. a variable gain amplifier having control :grid and plate electrodes for amplifying said voltage, said amplier having an output voltage characteristic which is a logarithmic function of an :applied D.C. biasing potential, a power ampli- -`'iier coupled to said variable gain ampliner and .including output means for producing an audio sound output corresponding to said audio frequency voltage, noise responsive meanslfor producing an alternating voltage proportional to the amplitude of ambient noise in the vicinity of said sound 'output means, m'eans for rectifying said alternating voltage, athermionic amplifier tube having input and output electrodes, means to bias said thermionic amplifier to the portion of its characteristic curve at which it has a logarithmic response means for applying said rectied alternating voltage to said input electrodes, a bias rectiiier and a lter coupled to said output electrodes for producing a D.-C. bias voltage, said bias voltage varying as a logarithmic function of the amplitude of the ambient noise, and means for applying said biasvoltage to the control grid of said variable gain amplifier, thereby to vary the output power level of the sound output means as a linear function of the ambient noise level.

5. A device of the character described in claim 4 which includes a second rectifier in parallel with said bias rectifier and means for applying a predetermined biasy potential to said second rectiiier,

thereby to limit the amplitude of the biasing pol tential variation.`

LOWELL H. GOOD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,338,551 Stanko Jan. 4, 1944 2,382,848 Baumgartner Aug. 14, 1945 2,392,218 Anderson Jan. 1, 1946 2,409,977 I Fisher Oct. 22, 1946 2,420L933 Crawford et al May 2Q, 1947 2,449,971 Andrews Sept. 28, 1948 

